1. Field of the Invention
The present invention relates to an improved milling process and a roller mill. More specifically, this invention is directed to an improved milling process, wherein the material to be crushed is processed at a circular milling path under the action of at least one roller rotating along said path.
2. Description of the Prior Art
In conventional milling processes and roller mills of the above type the material to be crushed is fed through a peripheral opening situated above the milling path, as e.g. shown in the Swiss patent publications Nos. 558,678 and 406,795. It enters the milling chamber at one side, is partly seized by the rollers and partly falls down through the rollers, without having been contacted by one of them, into a collecting trough. From there the material is thrown upwards into the milling path by the action of rotating shovels, as can be seen from Swiss patent No. 406 795. An air current flowing through the mill chamber carries away the fine material. In mills of this type, as they also are shown in U.S. Pat. No. 1,499,516 and U.S. Pat. No. 3,955,766, the material to be crushed is thrown around in an uncontrolled way within the milling chamber. In addition to the newly fed material, already crushed pieces which still are in the milling chamber repeatedly are thrown into the milling path such that the milling rollers act upon a mixture of fine and coarse pieces which considerably reduces the efficiency of the milling process due to the necessary displacement of the fine material and the internal friction of the material under the influence of the rollers. The one-sided feeding of the material and the uncontrolled flow of the same within the milling chamber lead to extensive wear, especially of the stationary parts in the milling chamber.
In the U.S. Pat. No. 1,499,624 another mill of the mentioned type is disclosed, wherein said drawbacks are
avoided by means of a violent air current which, outside the milling chamber, carries back both the crushed material and the pieces which had not been subjected to the action of the rollers to the inlet of the mill such that all pieces above a certain grain size are again introduced into the mill together with the new material. This mill, however, has the drawback, that the material fed enters the milling path in an uncontrolled way, which results in a poor efficiency, since it is necessary to feed back the material several times by means of said energy consuming air current until the desired grain size is achieved.
Finally, from U.S. Pat. No. 2,189,441 a mill is known, wherein the supply of the material to be crushed into the milling path of the rollers proceeds in a controlled manner. The material enters the milling path distributed over the whole circumference and is subjected there to the action of the rollers. However, no means are provided to carry the crushed particles out of the milling path, the influence of the adhesive forces on them normally being bigger than the gravitational forces. Therefore, in this mill a relatively thick material layer builds up on the milling path during operation due to reduced removal of the crushed material from this area. As a consequence uncontrolled material flow will be caused due to turbulences partly annulling the effect of controlled supply. Furthermore, the efficiency of the milling action will be reduced due to internal friction in the layer on the milling path.